Equine encephalosis

Equine encephalosis

P G HOWELL, A J GUTHRIE AND J A W COETZER

Introduction

Equine encephalosis (EE) is the name assigned to a mild or subclinical orbivirus infection of equids. The virus is transmitted by species of Culicoides, and as a result the epidemiology has much in common with African horsesickness (AHS). The designated name of the infection is unfortunately a misnomer since, by definition, neither lesions nor dysfunctions of the brain are characteristic features of the disease.

The first isolate of equine encephalosis virus (EEV) was recovered in March 1967 in South Africa from the blood and tissues of a 13-year-old Thoroughbred mare that was euthanased after showing a peracute nervous derangement. It was reported that two other mares on the same stud farm became ill during the following few days. One died, while the second recovered after a convalescence of 14 days. Although it was recorded that virus was also recovered from blood samples taken from horses that had exhibited no clinical signs of disease except for a fever, virus was also recovered from the organs of horses that had died in other parts of South Africa.4

Aetiology

Equine encephalosis virus is classified as an Orbivirus in the family Reoviridae and shares many morphological and physiochemical characteristics with other orbiviruses (see Bluetongue, and African horse sickness.)

Field samples of blood and tissues from infected horses have provided seven valid serotypes (Table 106.1), which on cross-neutralization tests have shown insignificant crossreactivity between heterologous antigens and antisera.5, 6, 8 Antisera produced in sheep have given unequivocal speci- ficity in the routine identification and classification of all field isolates recovered to date.

In a transmission electron microscopic study undertaken on infected BHK21/C13 cells, it was found that the negatively stained virus particles closely resemble those of AHS and bluetongue (BT).10 Examination of thin sections of infected cells revealed a replication cycle in the cytoplasm in the perinuclear area, with progeny particles visible after 28 hours. Irregularly shaped, granular inclusion bodies without a membrane in which maturing virus particles embedded in the matrix were observed became evident in the perinuclear region. The aggregations of virus particles in the cytoplasmic matrix resemble large crystals in which sectioned particles have a diameter of 73 nm. It was concluded that the observations suggested that the capsid is composed of 32 morphological subunits, as described for bluetongue virus (BTV),3 and represent subviral particles, while the larger ill-defined particles in the crystalline bodies are the complete virion with an outer diffuse polypeptide layer.

Table 106.1 Classification, identification, source and origin of currently recognized serotypes of equine encephalosis virus

SEROTYPE IDENTIFICATION SOURCE ORIGIN (YEAR) REFERENCE
1 Bryanston, M8/76 Foetal liver/spleen Colesberg, Western Cape (1976) 5
2 Cascara Organ suspension Kimberley, Northern Cape (1967) 4
3 Gamil, M9/71 Blood Naboomspruit, Limpopo Province (1971) 5
4 Kaalplaas, 7088 (7–2) Blood Onderstepoort, Gauteng (1974) 5
5 Kyalami, 7084 (12–3) Blood Johannesburg, Gauteng (1974) 11
6 Potchefstroom, Else EP8/91 Blood Potchefstroom, North West Province (1991) 6
7 E21/20 Blood St. Lucia – KwaZulu-Natal (2000) 8

Equine encephalosis virus replicates in BHK21/C13 monolayers and produces a distinctive cytopathology when the monolayers are viewed under low-power light microscopy. Affected monolayers show a fine-textured cytopathic effect in contrast to the development ofmore focal refractile cells which rapidly become detached when such monolayers are infected with AHSV. Lines of Vero (ATCC CCL81) cells vary in their susceptibility to EEV. Plaque assay may be undertaken in susceptible lines: plaques become visible after five to six days’ incubation. Serotype 1 of EEV produces high yields with no cytopathic effect in C6/36 (Aedes albopictus) cell cultures.

The replication of EEV is unaffected by exposure to 5-bromodeoxyuridine at a concentration of 30 µg/ml, whereas actinomycin D at a concentration of 0,01 µg/ml reduces the yield of virus a hundred-fold. Exposure to a pH of 3 for one hour at 37 °C produces a considerable drop in the infectivity titre, whereas chloroform at a final concentration of 5 per cent has no effect on viral infectivity.4

The molecular characterization of EEV and an early attempt to develop genetic probes to distinguish between EEV and AHSV have been reported.17 The serotype 2 virus that was used in this study was partially purified by a method using Triton X1009 after which the dsRNA was extracted.16 Electrophoretic separation on a PAGE gel confirmed that the genome is composed of ten segments, which show some differences from the separation profiles of BTV and AHSV. Seven structural capsid proteins were separated on SDSPAGE gels representing four major and three minor polypeptides. These fractionation patterns closely resemble those of two other selected orbiviruses, namely BTV and AHSV. Incomplete clones of six of the dsRNA segments of EEV serotype 2 were obtained. From hybridization techniques it was established that segment 2 encodes for the serotype-specific antigen. To date the nucleotide sequence of the ten...

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